Medical electrodes are commonly used in two broad applications, namely, iontophoretic transdermal drug delivery and medical diagnostics by electrochemical methods. In the area of iontophoretic transdermal drug delivery, electrodes are the key elements in an electrochemical device that drives charged drug ions through the skin by an electrical current. "Iontophoresis: Fundamentals" by O. Wong, Cygnus Therapeutic Systems, Redwood City, Calif. provides an overview of iontophoretic transdermal drug delivery technology, and "Noninvasive Sampling of Biological Fluids by Iontophoresis" by P. Glickfeld et al., Pharmaceutical Research, Vol. 6, No. 11. 1989, describes the use of iontophoretic techniques to extract biological molecules from a human body for medical diagnosis. The content of the articles are incorporated herein. In the area of medical diagnostics, conventional Ag/AgCl electrodes serve as transducers that convert electrochemical signals derived from the human body to electronic signals which measures/monitors body or organ functions by conventional electronic instruments, such as electrocardiograph (EKG), electroencephalograph (EEG) and blood sensors. There is a significant difference between conventional Ag/AgCl electrodes and iontophoretic electrodes. Iontophoretic electrodes undergo extensive compositional changes within the electrodes after use; while there is little change in conventional Ag/AgCl electrodes after use. Silver and silver/silver chloride polymer thick film (PTF) printing inks for iontophoretic electrodes having high electrode capacity are needed to sustain long-duration (&gt;24 hours) drug delivery. An iontophoretic drug delivery device comprises an electrochemical cell consisting of a donor electrode coated with a drug/hydrogel mixture, a counter electrode coated with hydrogel, and an electrical current supply. The electrodes are adhered to a patient's skin and then a low electric current, typically &lt;0.2 miliampere/cm2 (mA/cm.sup.2) in current density, is applied to the device. Charged drug species are driven through the skin by the electric field between the two electrodes. Accompanying the electric current, an oxidation reaction at the anode and a reduction reaction at the cathode take place to maintain the electrons/ions balance within the electrochemical cell. In the case of Ag/AgCl PTF electrodes, Ag is oxidized to AgCl at the anode and AgCl is reduced to Ag at the cathode. As Ag and AgCl are depleted in the electrodes, the respective electrode potentials increase to high values which may lead to harmful side reactions that render these electrodes unsuitable for further iontophoretic drug delivery and limits the full utilization of Ag in the anode. Investigations were conducted to look into the causes of deficiencies in the presently known iontophoretic electrode materials, such as a silver foil or polymer silver/silver chloride composites. When tested in an electrochemical cell, these electrodes were found to quickly form a thin layer of silver chloride as a result of electrochemical oxidation of silver on the anode surface when an electric current passed through the cell. This insulating silver chloride layer leads to increased electrode potential and also hinders further electrochemical oxidation of silver, and eventually high electrode potential renders the device unusable for drug delivery when harmful side reactions occurs. This problem of Ag-to-AgCl conversion limited at the electrode surface is due to the inability of the chloride ion to penetrate inside the anode. A hydrophobic electrode surface and a hydrophobic polymer binder matrix of a polymer thick film (PTF) electrode coating are the two major barriers that hinder chloride ion transport into the anode for sustaining the Ag-to-AgCl conversion throughout the thickness of the electrode PTF coating.
Therefore choosing the proper anode and cathode materials is critical to the success of achieving the best efficiency coupled with low cost for an iontophoretic device. U.S. Pat. No. 4,752,285 to Petelenz et al. and U.S. Pat. No. 4,747,819 to Phills et al. disclose iontophoretic devices with silver or lead metal anodes and silver chloride cathodes. U.S. Pat. No. 5,147,297 to Myers et al. discloses an iontophoretic device using electrodes composed of hydrophobic polymers, conductive fillers and chemical species, such as silver and silver chloride, capable of undergoing oxidation-reduction reactions. However, these electrodes suffer from several deficiencies, such as: (a) difficult to assemble into patches that can conform to the contour of human body, and (b) limited capacities to sustain the electric current for long drug delivery duration due to the limited capacity of anode materials. U.S. Pat. No. 3,662,745 to Cosentino discloses EKG/EEG type of electrodes prepared by coating a Ag/AgCl paste containing hydrophobic and hydrophilic polymers, alumina powder, and silver and silver chloride powders onto an electrically conductive substrate to improve electrode sensitivity for electrochemical measurements. These electrodes suffer from low conductivity and low capacities for sustaining an electric current which is needed for a long duration in iontophoretic drug delivery systems. U.S. Pat. No. 4,371,459 to Nazarenko et al. and U.S. Pat. Nos. 4,877,512 and 5,051,208 to Bowns et al. discloses silver and silver/silver chloride conductive PTF compositions containing hydrophobic polymers. These Ag and Ag/AgCl conductive compositions were developed for flexible circuits and EKG, EEG electrode applications, and were found to suffer from deficiencies described above when used as electrodes for an iontophoretic device. An attractive approach to overcome the deficiencies mentioned above is to print electrodes with conductive polymer thick film (PTF) materials on a plastic film substrate to make flexible electrodes that can then be easily assembled into convenient transdermal patches. The present invention provides conductive Ag or Ag/AgCl PTF compositions for medical electrodes which meet this need and the need for long-term iontophoretic drug delivery. In addition, the present invention overcomes the deficiencies in existing iontophoretic drug delivery systems by: (a) efficiently utilizing the oxidation-reduction species to achieve low cost, (b) obtaining high capacity for long-duration drug delivery, (c) exhibiting screen printability, and (d) displaying good adhesion to plastic flexible film substrates.